Method and apparatus for measuring pressure inside a fluid system

ABSTRACT

A disclosed method determines fluid pressure inside a vessel without compromising the integrity of the vessel. A sensor is positioned in operative communication with the external wall of the vessel such that expansion of the external wall of the vessel exerts a force against the sensor that is directed substantially radially outward with respect to the vessel. A substantially radially inward force is caused to be directed against the sensor in response to the substantially radially outward force exerted by the external vessel wall. The sensor can thus be used to detect the magnitude of the substantially radially outward force. 
     A disclosed apparatus determines fluid pressure inside a vessel without compromising the integrity of the vessel. The apparatus includes a sensor and a band operatively associated with the sensor and configured to at least partially encircle the vessel so as to retain the sensor in operative communication against the external wall of the vessel.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of a provisional U.S. patentapplication filed by CardioMEMS, Inc. on Oct. 20, 2006, and identifiedby attorney docket number 042-20, and of provisional U.S. PatentApplication Ser. No. 60/858,308, filed Nov. 9, 2006.

TECHNICAL FIELD

The present invention relates generally to methods and apparatus formeasuring hydraulic or pneumatic pressure and relates more specificallyto a method and apparatus for measuring dynamic or static hydraulic orpneumatic pressure inside a fluid system by monitoring changes in stressor pressure states on the outside of a fluid system.

BACKGROUND OF THE INVENTION

In many industrial and biological environments, it is desirable todetermine the characteristics of fluid internal to fluid systems withoutbreaching the fluid-containing vessel. For example, it is desirable tomeasure blood pressure characteristics at various points in the organsand blood-carrying vessels without having to breach an organ or vesselsurgically to place a pressure sensor directly in the fluid containedtherein.

As one example, the assessment of patients with congestive heart failure(CHF) following cardiac surgery remains difficult. For up to six monthsfollowing surgery, these patients undergo a complex shift in theirfluid-volume status. In the outpatient setting, the management of thesepatients has been performed by a history of increasing shortness ofbreath and a physical examination entailing assessment of the extent ofpedal edema. Currently, these measurements are indirect surrogates of apoorly functioning heart and do not provide objective data on cardiachemodynamics including heart filling pressures and cardiac output.Another test commonly used to evaluate CHF is a chest X-ray.Unfortunately, this test also does not provide objective hemodynamicdata. A Swan-Ganz catheter does provide cardiac hemodynamics and isroutinely utilized during and immediately following cardiac surgery.However, it is unreasonable to perform this procedure on a routine basisin the outpatient setting for the necessary adjustment of medicationsrelated to CHF because of the danger and discomfort to which the patientis subjected. To date, there is very little non-invasive objectivehemodynamic or cardiodynamic data following cardiac surgery that guidesthe proper management in this complex group of patients.

The availability of an implantable device with the capability of safe,non-invasive hemodynamic monitoring has the potential to change thelandscape in the management of patients following cardiac surgery.Availability of such a device would greatly enhance CHF management,patient lifestyle and reduce unnecessary hospitalizations and costs tosociety, currently estimated at $38 billion/year. Moreover, thepotential for life-long in-hospital and outpatient monitoring of thesepatients may significantly decrease risks associated with invasivehemodynamic monitoring and allow more succinct tailoring of heartfailure medications. Furthermore, with a portable monitoring system,patients can be monitored at home, negating costs associated withreadmissions and emergency room visits for patients with heart failure.

SUMMARY OF THE INVENTION

The assessment of pressure inside a fluid system by monitoring changesin stress or pressure states on the exterior of a fluid system (e.g.,fluid carrying pipe or tubing, pressure vessel, organ, or a bloodvessel-collectively referred to hereafter as a “vessel”) using awireless pressure sensor provides a valuable tool for sensing pressurenon-invasively in harsh industrial environments or inside the human bodywithout the need to open the fluid-containing vessel or organ. Byeliminating the need to breach the fluid containing vessel,installation, be it mechanical or surgical, is greatly simplified. Thereis no risk of thromboembolitic event, either through clotting if aforeign body is placed in the fluid, or through the embolization of aforeign body (e.g., a sensor or piece of a sensor) as there would be ifthe sensor were placed within the vessel and directly in the fluid.

Stated somewhat more specifically, in a first aspect the presentinvention comprises a method for determining fluid pressure inside avessel without compromising the integrity of the vessel. A sensor ispositioned in operative communication with the external wall of thevessel such that expansion of the external wall of the vessel exerts aforce against the sensor that is directed substantially radially outwardwith respect to the vessel. A substantially radially inward force iscaused to be directed against the sensor in response to thesubstantially radially outward force exerted by the external vesselwall. The sensor can thus be used to detect the magnitude of thesubstantially radially outward force.

In a second aspect, the present invention comprises an apparatus fordetermining fluid pressure inside a vessel without compromising theintegrity of the vessel. The apparatus includes a sensor and a bandoperatively associated with the sensor and configured to at leastpartially encircle the vessel so as to retain the sensor in operativecommunication against the external wall of the vessel. The band exerts asubstantially radially inward force against the sensor in response tothe substantially radially outward force exerted by the external vesselwall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a blood vessel of a human patient.

FIG. 2 is a cross-sectional view of the blood vessel of FIG. 1 showing asensor located to sense pressure through the vessel wall.

FIG. 3 is a cross-sectional view of a vessel having a sensor secured tothe vessel wall by means of a band.

FIG. 4 is a cross-sectional view of a vessel having a sensor secured tothe vessel wall by means of an alternate embodiment of a band.

FIG. 5 is a cross-sectional view of a vessel having a sensor secured tothe vessel wall by means of a further embodiment of a band in which twoband elements act as levers.

FIG. 6 is a cross-sectional view of a vessel having a sensor secured tothe vessel wall by means of a band having a cradle for receiving thesensor.

FIG. 7 is a cross-sectional view of an artery having a sensor sutured tothe outer layers of the artery without penetrating the intima.

FIG. 8 is a cross-sectional view of an aneurysm having a sensor suturedto the outer vessel wall.

FIG. 9 is a cross-sectional view of an artery in which a sensor issecured just below the visceral pericardium but outside the artery.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

Referring now in more detail to the drawings, in which like numeralsindicate like elements throughout the several views, FIG. 1 illustratesa blood vessel in the form of an artery 10 of a human patient. Theartery 10 is composed of three layers, the innermost being the intima12, the middle being the media 14, and the outermost being theadventitia 16. The adventitia 16 is easily dissected from the innerlayers (i.e., the media 14 and the intima 12).

As shown in FIG. 2, to determine hydrostatic pressure inside the artery10, a sensor 20 is positioned just below the adventitia 16 but outsidethe media 14. Securing the sensor 20 immediately below the adventitia 16and above the media 14 can be accomplished surgically by exposing thewall of the vessel 10, using a sharp object to make an incision throughthe adventitia 16 to the media 14. Then, using forceps to hold theincision open, the physician inserts the sensor 20 between theadventitia 16 and media 14. The sensor 20 can be pushed in between thelayers, making its own space. Then, the incision is closed. The sensorpackaging is biocompatible, and the vessel 10 heals around it, thussecuring the sensor 20 without the need for additional securing means.If the sensor 20 remains securely placed and is subjected to reasonablyconstant stresses (other than changes in fluid pressure), the need forrecalibration over time can be avoided. Then, fluid pressure can bedetermined by interrogating the sensor.

The stresses in the vessel wall change as a result of the fluidcharacteristics of the internal fluid. In turn, the electricalcharacteristics of the sensor 20 are influenced by the stresses in thevessel wall. When interrogated, the wireless sensor 20 transmits an RFsignal to an external receiver that corresponds to a fluid pressure. Aninterrogating scheme such as that described in U.S. patent applicationSer. Nos. 11/276,571 and 11/105,294 can be used to determine and trackchanges in these electrical characteristics, correlating them,ultimately, to pressure and other characteristics of the fluid (e.g.,output values, temperature).

Pressure sensors suitable for the present application include, but arenot limited to, sensors such as those described in U.S. patentapplication Ser. No. 10/054,671, filed on Jan. 22, 2002; U.S. patentapplication Ser. No. 10/886,829, filed on Jan. 22, 2002; U.S. patentapplication Ser. No. 10/215,377, filed on Aug. 7, 2002; U.S. patentapplication Ser. No. 10/215,379, filed on Aug. 7, 2002; U.S. patentapplication Ser. No. 10/943,772, filed on Sep. 16, 2004; U.S. patentapplication Ser. No. 11/157,375, filed on Jun. 21, 2005; U.S. Pat. No.6,926,670; U.S. Pat. No. 6,926,670; and U.S. Pat. No. 7,073,387, all ofwhich are hereby incorporated by reference in their entireties.

As an example of a sensor 20 that is suitable for the present invention,a sensor includes a housing defining a chamber and having a deflectableexterior wall portion that serves as a “sensing surface.” Capacitor andinductor elements within the chamber form an LC circuit. A firstcapacitor element is coupled to the deflectable wall portion. As thewall deflects in response to changes in ambient pressure, the firstcapacitor is displaced with respect to the second capacitor, therebychanging the capacitance of the circuit. The resonant frequency of theLC circuit thus changes. This change in the resonant frequency of the LCcircuit can be detected by interrogating the sensor. More specifically,the sensor is electromagnetically coupled to an external transmitter,which induces a current in the LC circuit that oscillates at theresonant frequency of the sensor. This oscillation causes a change inthe frequency spectrum of the transmitted signal. From this change, thebandwidth and resonant frequency of the sensor can be determined, and anassociated pressure can be established.

The sensor 20 is placed in operative communication with the externalwall of a fluid-carrying vessel, either by placing the sensing surfaceof the sensor directly against the external wall of the vessel or byplacing the sensing surface against an intervening element thattransmits pressure changes from the vessel wall to the sensing surface.The sensor 20 is thus able to detect changes in pressure of the fluidflowing within the vessel. In the context of a biomedical application,the sensor 20 can be placed in conductive communication with the wall ofa blood vessel. When the blood pushes on the wall of the vessel, aradially outward force is exerted by the vessel wall. The sensor sensesthe change in pressure. The tissue backing the sensor exerts a radiallyinward force in opposition to the radially outward force exerted by thevessel wall. The sensor can be embedded within the vessel wall, fixeddirectly to the outside of the vessel wall, or fixed to a layer oftissue (e.g., fat) that is in direct contact with the vessel wall.

Polymer-metal, polymer-ceramic, biological tissue-metal and biologicaltissue-ceramic interfaces of the vessel wall-sensing surface areparticularly suited to the present application.

FIG. 3 illustrates an alternative embodiment in which a sensor 30 issupplied with a band 32 to secure it to the periphery of a vessel 34.The band 32 of the disclosed embodiment is manufactured from a materialthat is flexible and substantially nonextensible, for example, abiocompatible polymer (e.g., PTFE). The band 32 has two band portions35, 36. Each band portion is joined at one end to the sensor 30. Thevessel 34 is exposed, and a sensing surface 40 of the sensor 30 isplaced in contact with the outer vessel wall 42. The ends 37, 38 of thepolymer band portions 35, 36 are wrapped around the vessel and securedtogether with a suture 44 or other suitable fastener (e.g., adhesive,snap, button, staple, etc.) to anchor the sensor 30 to the vessel 34.When the vessel wall exerts a radially outward force in response to anincrease in fluid pressure within the vessel, the band 32 exerts aradially inward force against the sensor 30. The increase in forcecauses changes to the electrical characteristics of the sensor, andthese electrical characteristics can be detected and correlated topressure changes within the vessel.

In the alternative, the band 32 can be formed as a single, continuouscomponent to which the sensor is attached, rather than two separate bandportions, with the free ends of the band 32 wrapped around the vesseland secured.

FIG. 4 depicts another embodiment, in which a sensor 50 is held inposition against the wall of a vessel 52 by a C-shaped partial ring 54.The pliable vessel 52 is worked through the opening of the partial ring54. Alternately the partial ring 54 can be fabricated from ashape-memory metal or alloy (e.g. nitinol). In this latter case, thepartial ring 54 can be configured to close around the vessel 52 by shapememory effects after the ring has been implanted.

FIG. 5 illustrates a further embodiment, in which two metal bands 60, 62are used to create a lever by which a change in the fluid pressurewithin a vessel 64 is communicated to a sensor 66. A first band 60 issecured to the side of the sensor 66 opposite the surface in contactwith the vessel wall, and the second band 62 is secured to the side ofthe sensor 66 in contact with the vessel wall. When the vessel 64changes diameter with a change in blood flow, the first and second bands60, 62 pivot and act as a lever to compress or expand, thereby changingthe electrical characteristics of the sensor 66.

FIG. 6 shows a band 70 comprising a cradle 72 in which a sensor 74 canbe placed. The cradle 72 surrounds the top and sides of the sensor 74,leaving only the sensing surface 76 of the sensor exposed. The cradle 72eliminates the need to recalibrate the sensor 74 if the stress exertedon the sensor from the environment surrounding the non-sensing surfacechanges appreciably over time.

Referring now to FIG. 7, in yet another embodiment a sensor 80 issutured to the outside of a vessel 82. The sutures 84 are superficialand do not puncture the vessel or organ (e.g., if secured to an artery,the stitches do not puncture the intima).

If the sensor 80 were only sutured to the vessel 82, without more, thesensor would simply move with the vessel wall as it expands orcontracts. However, the sensor is backed by surrounding tissue 86, e.g.,muscle or fat, which exerts a radially inward force with respect to thevessel 82 as the vessel expands or contracts. (It will be understoodthat the vessel 82 is completely surrounded by tissue 86, but only aportion of the tissue is shown in FIG. 7 for clarity of illustration).Thus the sensor housing is prevented from substantial movement, and thesensing surface of the sensor can measure the applied force.

As yet another alternative embodiment, shown in FIG. 8, an adhesive canbe used to secure a sensor 90 to the outside of the vessel. For example,a sensor 90 can be secured via a biocompatible adhesive to the exteriorof a radial artery or to the exterior of an aneurysm 92. Again, thesensor is backed by surrounding tissue (not shown) and is prevented fromsubstantial radially outward movement when the aneurysm 92 attempts toexpand. The sensing surface 94 of the sensor 90 is thereby capable ofdetecting the applied force resulting from an increase in pressureinside the aneurysm 92.

Another example of using a sensor to determine hydrostatic pressureinside an artery in a human patient is to secure the sensor just belowthe visceral pericardium but outside the artery as shown in FIG. 9. Thevisceral pericardium 100 is the inner layer of the pericardium, aconical sac of fibrous tissue that surrounds the heart and the roots ofthe great blood vessels. Securing the sensor 102 immediately below thevisceral pericardium 100 and above the artery 104 can be accomplishedsurgically by exposing the visceral pericardium, using a sharp object tomake an incision through the visceral pericardium to the wall of thevessel 104, using forceps to hold the incision open and the blunt end ofclosed scissors to dissect a pocket under the visceral pericardium, andinserting the sensor 102 between the artery 104 and visceral pericardium100. Then, the incision is closed. This method is extremely advantageousbecause the artery wall is not breached, even in part.

In each of the foregoing biological applications, the sensor packagingis biocompatible, and the vessel heals around it, thus securing thesensor without the need for additional securing means. The sensorremains securely placed, and reasonably constant stresses (other thanfluid pressure) are exerted on the sensor to avoid the need forrecalibration over time. Then, fluid pressure can be determined byinterrogating the sensor as previously explained.

In the above biological examples, the sensors can be delivered andsecured to the vessel by transcatheter delivery or open surgicaltechniques. These techniques are well within the level of skill of thosepracticed in the art, requiring only standard surgical procedures.

Outside of the realm of medicine, the same pressure sensors can be usedas described above to sense stress on the exterior of vessels todetermine the pressure and other characteristics of the fluid. Thesensor could be installed on vessels that contain fluids and thepressure within those vessels monitored wirelessly. This is useful inharsh industrial environments (e.g., extreme temperatures, dangerouschemical environments). The sensors described in U.S. Pat. Nos.6,111,520 and 6,278,379, incorporated in their entireties by reference,are particularly suitable for such industrial applications.

Furthermore, this invention can be practiced using a multitude ofwireless sensing schemes, such as ultrasonic sensing.

Finally, it will be understood that the preferred embodiment has beendisclosed by way of example, and that other modifications may occur tothose skilled in the art without departing from the scope and spirit ofthe appended claims.

1. A method for determining fluid pressure inside a vessel withoutcompromising the integrity of the vessel, comprising the steps of:positioning a sensor in operative communication with the external wallof the vessel such that expansion of the external wall of the vesselexerts a force against the sensor that is directed substantiallyradially outward with respect to the vessel; causing a substantiallyradially inward force to be directed against the sensor in response tothe substantially radially outward force exerted by the external vesselwall; and using the sensor to detect the magnitude of the substantiallyradially outward force.
 2. The method of claim 1, wherein the step ofcausing a substantially radially inward force to be directed against thesensor in response to the substantially radially outward force exertedby the external vessel wall comprises the step of securing the sensor tothe vessel with a band that at least partially encircles the vessel. 3.The method of claim 2, wherein the step of securing the sensor with aband that at least partially encircles the vessel comprises the step ofsecuring the sensor with a band that completely encircles the vessel. 4.The method of claim 1, wherein the step of causing a substantiallyradially inward force to be directed against the sensor in response tothe substantially radially outward force exerted by the external vesselwall comprises the step of inserting the sensor between the externalwall of the vessel and an adjacent body of tissue.
 5. The method ofclaim 4, wherein the vessel is an artery, and wherein the step ofinserting the sensor between the external wall of the vessel and anadjacent body of tissue comprises the step of inserting the sensor justbelow the adventitia of the artery but outside the media of the artery.6. The method of claim 4, wherein the vessel is an aneurysm.
 7. Themethod of claim 4, wherein the vessel runs through the visceralpericardium, and wherein the step of inserting the sensor between theexternal wall of the vessel and an adjacent body of tissue comprises thestep of inserting the sensor outside the vessel and within the visceralpericardium.
 8. An apparatus for determining fluid pressure inside avessel without compromising the integrity of the vessel, wherein anincrease in fluid pressure within the vessel causes the external wall ofthe vessel to exert a radially outward force, the apparatus comprising:a sensor; and a band operatively associated with the sensor andconfigured to at least partially encircle the vessel so as to retain thesensor in operative communication against the external wall of thevessel; wherein the band exerts a radially inward force on the sensor inopposition to the radially outward force exerted by the external wall ofthe vessel.
 9. The apparatus of claim 8, wherein the band completelyencircles the vessel.
 10. The apparatus of claim 8, wherein said bandcomprises a pair of band portions, each having a first end secured tothe sensor, and a second end configured to at least partially encirclethe vessel.
 11. The apparatus of claim 8, wherein the band is flexibleand substantially non-extensible.
 12. The apparatus of claim 8, whereinthe band is substantially rigid.
 13. The apparatus of claim 8, whereinthe band is formed from a shape-memory material.
 14. The apparatus ofclaim 8 wherein the sensor comprises a sensing surface, and wherein theband is configured to retain the sensing surface of the sensor directlyagainst the external wall of the vessel.
 15. The apparatus of claim 8wherein the sensor comprises a sensing surface, and wherein the band isconfigured to retain the sensing surface of the sensor against anintervening element that is in contact with the external wall of thevessel.